# std::atomic<T>::wait

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< cpp‎ | atomic‎ | atomic

void wait( T old, std::memory_order order = std::memory_order_seq_cst ) const noexcept; | (1) | (since C++20) (constexpr since C++26)
void wait( T old, std::memory_order order = std::memory_order_seq_cst ) const volatile noexcept; | (2) | (since C++20)

Performs atomic waiting operations. Behaves as if it repeatedly performs the following steps:

- Compare the value representation of this->load(order) with that of old.
  - If those are equal, then blocks until *this is notified by notify_one() or notify_all(), or the thread is unblocked spuriously.
  - Otherwise, returns.

These functions are guaranteed to return only if value has changed, even if underlying implementation unblocks spuriously.

If order is not std::memory_order_relaxed, std::memory_order_consume, std::memory_order_acquire or std::memory_order_seq_cst, the behavior is undefined.

### Parameters

old | - | the value to check the `atomic`'s object no longer contains
order | - | memory order constraints to enforce

### Notes

This form of change-detection is often more efficient than simple polling or pure spinlocks.

Due to the ABA problem, transient changes from old to another value and back to old might be missed, and not unblock.

The comparison is bitwise (similar to std::memcmp); no comparison operator is used. Padding bits that never participate in an object's value representation are ignored.

### Example

Run this code

    #include <atomic>
    #include <chrono>
    #include <future>
    #include <iostream>
    #include <thread>

    using namespace std::literals;

    int main()
    {
        std::atomic<bool> all_tasks_completed{false};
        std::atomic<unsigned> completion_count{};
        std::future<void> task_futures[16];
        std::atomic<unsigned> outstanding_task_count{16};

        // Spawn several tasks which take different amounts of
        // time, then decrement the outstanding task count.
        for (std::future<void>& task_future : task_futures)
            task_future = std::async([&]
            {
                // This sleep represents doing real work...
                std::this_thread::sleep_for(50ms);

                ++completion_count;
                --outstanding_task_count;

                // When the task count falls to zero, notify
                // the waiter (main thread in this case).
                if (outstanding_task_count.load() == 0)
                {
                    all_tasks_completed = true;
                    all_tasks_completed.notify_one();
                }
            });

        all_tasks_completed.wait(false);

        std::cout << "Tasks completed = " << completion_count.load() << '\n';
    }

Output:

    Tasks completed = 16

### See also

notify_one (C++20) | notifies at least one thread waiting on the atomic object (public member function)
notify_all (C++20) | notifies all threads blocked waiting on the atomic object (public member function)
atomic_notify_one (C++20) | notifies a thread blocked in atomic_wait (function template)
atomic_notify_all (C++20) | notifies all threads blocked in atomic_wait (function template)

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